Intelligent Objects: The Future of Simulation

C. Dennis Pegden
Simio LLC
504 Beaver St.
Sewickley, PA 15143, U.S.A.

ABSTRACT

This paper describes a new modeling system -- SimioTM that
is designed to simplify model building by promoting
a modeling paradigm shift from the process orientation to
an object orientation. Simio is a simulation modeling
framework based on intelligent objects. The intelligent
objects are built by modelers and then may be reused in
multiple modeling projects. Although the Simio framework
is focused on object-based modeling, it also supports
a seamless use of multiple modeling paradigms including
event, process, object, and agent-based modeling.

1 MODELING PARADIGMS

In the early days of discrete event simulation the dominant
modeling paradigm was the event orientation implemented
by tools such as Simscript (Markowitz, et .al
1962.) and GASP (Pritsker, 1967). In this modeling paradigm
the system is viewed as a series of instantaneous
events that change the state of the system. The modeler
defines the events in the system and models the state
changes that take place when those events occur. This
approach to modeling is very flexible and efficient, but is
also a relatively abstract representation of the system. As
a result many people found modeling using an event
orientation to be difficult.

In the 80's the process orientation displaced the event
orientation as the dominant approach to discrete event simulation.
In the process view we describe the movement
of passive entities through the system as a process flow.
The process flow is described by a series of process steps (e.g. Seize, Delay, Release) that model the state changes
that take place in the system. This approach dates back to
the 1960's with the introduction of GPSS (Gordon, 1960)
and provided a more natural way to describe the system.
However because of many practical issues with the original
GPSS (e.g. an integer clock and slow execution) it did
not become the dominant approach until improved versions
of GPSS (Henriksen, 76) along with newer process
languages such as SLAM( Pegden/Pritsker, 79) and
SIMAN (Pegden, 82) became widely used in the 80's.
During the 80's and 90's graphical modeling and
animation also emerged as key features in simulation
modeling tools. Graphical model building simplified the
process of building process models, and graphical animation dramatically improved the viewing and validation of
simulation results. The introduction of Microsoft Windows
made it possible to build improved graphical user
interfaces and a number of new graphically based tools
emerged (e.g. ProModel and Witness).

Another conceptual advance that occurred during this
time was the introduction of hierarchical process modeling
tools that supported the notion of domain specific
process libraries. The basic concept here is to allow users
to create new process steps by combining existing process
steps. The widely used Arena modeling system (Pegden/
Davis, 1992) is a good example of this capability.

Since the wide spread shift to a graphics-based
process orientation there have been refinements and improvements
in the tools, but no real advances in the underlying
framework. The vast majority of discrete event
models continue to be built using the same process orientation
that has been widely used for the past 25 years.

Although a process orientation has proven to be very
effective in practice, an object orientation provides an attractive
alternative modeling paradigm that has the potential
to be more natural and easier to use. In an object
orientation we model the system by describing the objects
that make up the system. For example we model a factory
by describing the workers, machines, conveyors, robots,
and other objects that make up the system. The system
behavior emerges from the interaction of these objects.

Although a number of products have been introduced
to support an object orientation, to date many practitioners
who have elected to stick with the process orientation.
A big reason for this is that while the underlying modeling
paradigm might be simpler and less abstract, the specific
implementation may be difficult to learn and use (e.g. require programming), or slow in execution. This is
no different than the challenges faced by the process
orientation in unseating the event orientation. Although
the first process modeling tool (GPSS) was introduced in
1961, it took 25 years before the process orientation was
developed to the point that practitioners were persuaded
to make the paradigm shift.
This paper describes Simio -- a new simulation modeling
tool that is designed to make the object orientation
easy to use and efficient to execute. Although Simio incorporates
a number of innovative features in pursuit of
this goal, only time will tell if this tool has bridged the
many practical issues that must be addressed to trigger a
widespread paradigm shift in the way practitioners build
models.

The tool is designed from the ground up to support
the object modeling paradigm; however it also supports
the seamless use of multiple modeling paradigms including
a process orientation and event orientation. It also
fully supports both discrete and continuous systems, along
with large scale applications based on agent-based modeling.
These modeling paradigms can be freely mixed within
a single model.